Piezoelectric crystal plate



Oct. 18, 1949. H. s. BAERWALD 2,485,130

PIEZOELECTRIC CRYSTAL PLATE Original Filed March 19, 1945 I2( s) NOT EQUiVALENT F'IG.I F162 ALL THREE PLATES E uwAuEnr C DOUBLE SERIES FIG. 3

INVENTOR. HANS (3. BAERWALD NEY Patented is, 1949', Q

UNITED STATES PATENT OFFICE 2,485,130 I I I PIEZOELECTRIC CRYSTAL PLATE Hans G. Baerwald, Cleveland Heights, Ohio, as-

signor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Original application March 19, 1945, Serial No.

1948, Serial No. 20,1'13

This application is a division of my application Serial Number 583,477, filed March 19, 1945, for Piezoelectric crystal plates, to which reference may be made for a complete discussion of the invention with respect to all crystal classes to which the invention is applicable.

This invention relates to cuts of piezoelectric crystals and more particularly to thickness-controlled shear crystal plates for frequency control or as filter elements in the upper radio and ultrahigh frequency range.

The fundamental object of this invention is to provide from crystal classes I2, C2v and Ca, and the cuts or orientations therein possible by virtue of the laws of phenomenological crystal physics, plates with the following three fundamental properties:

1. They are excitable in simple-mode thickness-controlled elastic shear vibrations by longitudinal electric fields and/or dielectric displacements;

2. No elastic and dielectric interaction of these shear modes with any other modes of vibration exists;

3. These orientations are independent of the physical constants of any particular material belonging to one of the specified classes of crystallographic symmetry as well as of extraneous physical conditions such as temperature. For the purpose of this description, cuts with these three properties are called piezoelectric unconditionally pure thickness-controlled shear cuts.

Another object isto provide piezoelectric unconditionally pure thickness-controlled shear cuts with two simple, but in general mutually different modes, frequency and piezoelectric coupling constants, and the modes being free from mutual interaction except via the longitudinal electrical component linked with both.

Another object is to provide, for one and the same crystal class, two piezoelectric unconditionally pure thickness-controlled shear cuts within general different, single, simple modes, frequency, and piezoelectric coupling constants.

For a better understanding of the present inventiomtogether with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In accordance with a feature of th invention there is provided a piezoelectric thickness-controlled shear crystal plate cut from crystalline material selected from one of the following crystallographic classes: I2; Ca; (In. These com- 4 Claims. (01. 171-327) Divided and this application April 10,

prise all piezoelectric classes characterized by the common properties that they'possess a crystallographic plane of symmetry and that all of their elements of crystallographic symmetry are of orders 'less than four. Such a plate is excitable in two independent harmonic series of unconditionally pure thickness-controlled shear vibrations; also, there are no other plates cut from any crystalline material which have'the same property. The plate which is cut from the above described crystalline material has apair of plane parallel major faces which are parallel to a crystallographic plane of symmetry.

With respect to the drawings, Fig. 1 shows the orientation of a piezoelectrically active crystal plate having thickness-controlled unconditionally pure shear modes of vibration for a material of the monoclinic class I2 (=05). Fig. 2 illustrates the corresponding conditions for the orthorhombic class C2v, and Fig. 3 illustrates the corresponding conditions for the dicyclio polar trigonal class 03v.

The element of lowest limited crystallographic symmetry capable of independent existence is that of an even-rank plane of symmetry (Z plane). It is characteristic of the monoclinic system. This symmetry eliminates all interactions between the normal elastic components and the shear component in the Z plane, on the one hand, and the two shear components in planes containing the Z axis, on the other hand. This represents a property of both a crystallographic mirror plane and a digonal rotation axis, which are even-rank equivalent, and is therefore independent of the location of the X and Y axes which are not defined by virtue of crystallographic symmetry. The only piezoelectric class in which the aforementioned two shear components ar excited by longitudinal fields is the class I2 whose only crystallographic element of symmetry is a plane (Z plane). A Z-cut plate in this class, as shown in Fig; 1, has two independent series of unconditionally pure thickness-controlled shear vibrations.

As far as even-rank properties are concerned, the orthorhombic system is identical with the monoclinic system, but simultaneously with respect to two mutually orthogonal axes, X and Y, which fact induces the same property with respect to the third, Z. Thus, each crystallographic axis is a direction of unconditionally pure double-series shear waves with directions of displacement unconditionally along the other two axes. There are two piezoelectric orthorhombic classes, V and Czvf In the former, whose crystallographic elements of symmetry are the three digonal axes. none of the aforementioned shear waves is excitable by longitudinal fields. In the latter, whose crystallographic elements of symmetry are two mirror planes, the X and Y planes, in combination with the digonal Z axis, the x and Y fields will excite one unconditionally pure shear component each, namely, that associated with the Z motion. There are therefore two series of unconditionally pure shear modes with directions of displacement along the Z axis in C2v, namely, the X cut and the Y out, each parallel to the respective crystallographic planes of symmetry, each with a single series of unconditionally pure shear vibrations of mutually independent frequency constants.

I The combination of a trigonal axis with a crystallographic plane of symmetry parallel to it is unique in the sense that it will not change the dielectric, piezoelectric and elastic properties associated with this plane when it is sole element of crystallographic symmetry, but will merely materialize this plane in three mutually equivalent positions rotated with respect to each other by 120 degrees. Combinations of a mirror plane with other elements of crystallographic symmetry, on the other hand, e. g., with an axis perpendicular to it or with an axis of the rotation or inversion-rotation type parallel to it will automatically induce higher elements of elasto-dielectric symmetry such as elimination of all piezoelectric couplings or degeneracy, etc., besides multiplying the number of equivalent positions of the plane. The reasons for this have been set out in detail in the parent application.

Therefore, the properties of the class Cav are for our purposes identical with those of the previously described 1: except for the three equivalent,

positions of the mirror plane. These three equivalent cuts are shown in Fig. 3. The three X axes are drawn perpendicular to these planes, according to the usual convention. It also en-' sues from the previous argument or in more detail from the parent application that, as was the case in the monoclinic and orthorhombic system,

, Cav represents the only class of the trigonal system from which plates can be cut in accordance with this invention. I

The classes I2, Czv and Ca are all of the piezoelectric classes whose crystallographic symmetry elements are of orders less than four and which possess a crystallographic plane of symmetry. The thickness-controlled shear crystal plate in each case is a plate having a pair of plane parallel major faces which are parallel to a crystallographic plane of symmetry.

As in the angular neighborhood of any pure shear cut undesired elastic interactions are proportional to the square of an angular deviation only and, therefore, are practically eliminated throughout that neighborhood, moderate angular deviations cannot only be tolerated but may even be desirable if they lead to a slight improvement in the temperature coeilicients.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modiflcations may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In a piezoelectric thickness-controlled shear crystal plate: a plate cut from crystalline piezoelectric material which has a crystallographic plane of symmetry and all of whose crystallographic symmetry elements are of orders less than four, said plate having a pair of plane parallel major faces which are substantially parallel to a crystallographic plane of symmetry.

2. In a piezoelectric thickness-controlled shear crystal plate: a plate cut from crystalline piezoelectric material of the class Ii, said plate having a pair of plane parallel major faces which are substantially parallel to the crystallographic plane of symmetry of said crystalline material.

3. In a piezoelectric thickness-controlled shear crystal plate: a plate cut from crystalline piezoelectric material of the class 02v, said plate having a pair of plane parallel major faces which are substantially parallel to a crystallographic plane of symmetry of said crystalline material.

4. In a. piezoelectric thickness-controlled shear crystal plate: a plate cut from crystalline piezoelectric material of the class C", said plate having a pair oi plane parallel major faces which are substantially parallel to a crystallographic plane of symmetry of said crystalline material.

HANS G. BAERWALD.

REFERENCES CITED The following references are of record in the tile of this patent:

Cady, W. G. Piezoelectricity," McGraw-Hill Book Co. N. Y. 1946, pages 19 and 20. 

